The present invention is directed to an improved method of preparing a sulfur-modified polychloroprene latex.
Sulfur-modified chloroprene polymers are well known. They are obtained by polymerization of chloroprene, possibly with another unsaturated monomer, in the presence of elemental sulfur. The polymer obtained has a structure which is strongly cross-linked via polysulfide bonds and is difficult to work. In order to obtain a product of increased plasticity, the polymer is partially degraded by means of suitable agents which produce a breaking of some of the polysulfide bonds. This operation is referred to as peptization. Suitable plasticizing agents are described in general in U.S. Pat. No. 2,234,215.
Methods of peptization in alkaline aqueous emulsion have been described, for instance in French Pat. Nos. 1,376,314; 1,392,446; 1,393,099; 1,398,602; 2,076,922 and 2,089,811, as well as in U.S. Pat. No. 3,318,832. In accordance with these methods, the peptization is carried out after the polymerization by treating the polymer latex with an alkylthiuram disulfide, combined or not with a xanthogen disulfide or a dialkyldithiocarbamate. The main disadvantage of these processes is the slowness of the peptization (requiring several hours at 40.degree. C.). The reaction proceeds during the treatment of the latex for the elimination of the residual monomer as well as during the phase of isolation of the rubber. Furthermore, the plasticity of the rubbers obtained varies in time, which represents a serious drawback for their use and their conversion into manufactured products.
It is also known, in accordance with a different technique, to increase the plasticity of the sulfur-modified chloroprene polymers by means of modifying agents or chain-transfer agents, such as xanthogen disulfides, iodoform or alkyl mercaptans, which are present during the polymerization and which act as molecular weight regulators. These methods are described, for example, in French Pat. No. 1,457,004 and U.S. Pat. No. 3,397,173. The modified copolymers obtained can still be peptized subsequently.
Finally, French Pat. No. 2,008,562 and its corresponding U.S. Pat. No. 3,808,183 describe a method of preparing polychloroprene modified with sulfur and xanthogen or iodoform in which the polymer latex is peptized instantaneously by means of a thiol.
It is, therefore, an object of the present invention to provide a method of preparing, in a single step, sulfur-modified chloroprene polymers without auxiliary modifying agents, in which the polymerization and the peptization are carried out directly during the polymerization and not after completion of the polymerization, as in accordance with the known methods.
It is also an object of the present invention to provide an improved method for preparing sulfur-modified polychloroprene latices.
Other objects will be apparent to those skilled in the art from the present description.